It is known from Kawai et al U.S. Pat. No. 4,109,287 that an anodic oxidation treatment is applied onto aluminum to form an anodic oxide film or layer having a great number of micro pores, the micro pores in the anodic oxide film thus formed are then packed with a magnetic substance by various manners to form a magnetic layer, and the magnetic material thus obtained is used as a magnetic recording medium.
The remarkable feature of the magnetic recording medium prepared by the above-described manner is in that the magnetic recording medium shows so-called "perpendicular magnetic properties" since the magnetic substance exist in the perpendicular micro pores of the anodic oxide film and in such a magnetic recording medium, recording density can be greatly increased as compared to conventional magnetic recording media of a horizontal magnetization system prepared by coating a conventional magnetic material on the surface of a substrate.
In the case of using the magnetic film or layer obtained by packing the micro pores of the anodic oxide film with a magnetic substance for a magnetic recording medium, it is desired that the surface of the anodic oxide film thus formed be physically ground or polished to provide a sufficiently smooth surface, the thickness of the anodic oxide film after polishing being at least 2 .mu.m for preventing the film from being broken by a magnetic head and obtaining sufficient magnetic properties for use with either a contact type magnetic head or a floating type magnetic head, so that the magnetic recording medium can exhibit proper magnetic properties for the kind of a magnetic head, these magnetic properties being homogneous throughout the whole surface of the magentic recording medium.
However, in an anodic oxide film obtained by a general anodic oxidation treatment which is conventionally employed, there is a problem that the magnetic properties of magnetic recording medium are inadequate since the pore diameter of the micro pores is small, whereby the surface density of the micro pores occupying the surface of the anodic oxide film is too small, and also the coercive force (Hc) is too high to perform recording and reproduction of the output is too small owing to the small pore diameter of the micro pores.
It is also known that the diameter of micro pores in the anodic oxide film largely changes according to the kind of electrolytic bath used for the anodic oxidation treatment of aluminum. For example, with a sulfuric acid bath which is most generally used as the anodic oxidation treatment bath for aluminum, the pore diamteter of the micro pores of the anodic oxide film thus formed is about 100 to 200 .ANG., while when a phosphoric acid bath is used, the pore diamter is about 500 to 1,000 .ANG.. Thus, as far as the diameter of micro pores is concerned, a large pore diameter is obtained by using a phosphoric acid bath, whereby a large surface density of micro pores can be obtained, but the anodic oxide film formed using a simple phosphoric acid bath is liable to become brittle and the anodic oxide film thus formed has a thickness of about 2 .mu.m only.
On the other hand, when an anodic oxidation treatment is applied to aluminum using a bath composed of sulfuric acid, oxalic acid, or sulfosalicylic acid or a bath mainly composed of the aforesaid acids, an anodic oxide film having a strong film quality and a sufficiently thick film thickness is obtained but the pore diamter of the micro pores is small and hence it is required to enlarge the diameter of the micro pores by a secondary treatment.
For enlarging the micro pores in the anodic oxide film, a method of performing a secondary electrolysis using a phosphoric acid bath has been already proposed. However, when performing the secondary electrolysis using a phosphoric acid bath, the bottom portions of the micro pores are generally extremely enlarged, a great number of branched micro pores being further formed at the bottom portions of the micro pores, and the extraordinarily growing state of the micro pores cannot be foreseen and the extent of the extraodinary growing is not constant. Accordingly, in the case of packing the micro pores with a magnetic substance in a subsequent step, desirable perpendicular magnetic properties are not obtained and also it is impossible to perform an adjustment for obtaining appropriate magnetic properties for various magnetic heads.
From the results of various experiments made by the inventors on the enlargement of the micro pores in the anodic oxide film it is found that if an aluminum material which was subjected to an anodic oxidation treatment by using the conventional treating bath is immersed in an acid or alkaline bath to chemically dissolve a part of the anodic oxide film i.e. the walls of the micro pores, the micro pores can be uniformly enlarged through their full length and that in the enlargement of the micro pores by such chemical dissolution the diameter of the micro pores can be relatively freely controlled by properly adjusting the dissoluting conditions.
However, when a magnetic recording medium is prepared by depositing a magnetic substance in the micro pores thus enlarged as described above, the deposited amount of the magnetic substance frequently becomes microscopically nonuniform and also when such a magnetic recording medium is used as a high-density magnetic recording medium, the output becomes nonuniform to cause error. Therefore, it has been required to pack the micro pores with a magnetic substance in a microscopically uniform state.